The present invention generally relates to processes for production of ethanol from energy crops. The present invention further relates to improvements in one or more aspects of ethanol production from energy crops including, for example, improved methods for fractionating liquefied mash, improved efficiency of conversion of energy crop cellulosic components to monosaccharides, and improved yield of ethanol from energy crops.
Ethanol and corresponding co-products may be produced from a variety of starch-containing energy crops, such as grain and tubers, using any conventional dry mill or wet mill fermentation process known in the art. For example, in some typical processes, dry mill ethanol production utilizes the starch portion of corn kernels (which typically comprises 70% by weight of the kernel) wherein the starch component is converted by enzymatic hydrolysis to sugars that are then fermented to form ethanol. The ethanol is recovered by distillation leaving a still bottoms fraction comprising high levels of cellulosic-based fiber and unconverted, inaccessible, starch bound thereto. The still bottom fraction may be processed to form distillers grain that is typically used as ruminant animal feed due to the high fiber content.
In conventional starch to ethanol processes, the energy crop (typically a grain such as #2 yellow dent corn, wheat, barley or milo) is ground and slurried with process water and/or backset to form a mash. α-amylase enzyme is added to liquefy the mash and begin hydrolyzing accessible starch to long chain dextrins. In some processes, the starch slurry is heated using a hydroheater or in a stirred tank to gelatinize granular starch, before or after adding the α-amylase enzyme, and the heated gelatinized slurry is forwarded to liquefaction tank to finish starch hydrolysis and form a liquefied mash. In some other processes, termed “non-cooking processes” the starch slurry is not heated. In any such process, additional α-amylase enzyme may be added to hydrolyze the gelatinized starch to short chain dextrins. The liquefied mash is then cooled to a temperature in the range of 30° C. to 35° C. and the pH is adjusted to within a range of 4 to 5 whereupon glucoamylase is added to form fermentable hexose monosaccharides (e.g., glucose) from dextrins. Yeast is then added to convert the hexose to ethanol. In some processes, glucoamylase and yeast are added simultaneously or in close sequence to form ethanol from glucose in a simultaneous saccharification and fermentation process (“SSF”).
Problematically, in such processes, cellulose contained in the fiber component and starch bound thereto (i.e., inaccessible starch) are not converted to fermentable sugar. Some prior art processes attempt convert cellulose and bound starch to fermentable sugar (i) by adding cellulase enzyme and, optionally, glucoamylase enzyme, to the mash during the liquefaction step or (ii) by adding cellulose enzyme to the liquefied mash during saccharification, fermentation or during SSF. This approach suffers from several drawbacks. For instance, the normal conditions for saccharification and fermentation (e.g., 30° C. to 35° C. and pH 4 to 5) are not optimal for cellulase enzyme activity. An alternative is to perform cellulose hydrolysis at the optimal conditions of from 50° C. to 55° C. and pH 5, but this option is costly because the entirety of the mash must be conditioned resulting in high capital costs. Moreover, extended cycle time and reduced throughput due to long mixing and residence times required to effective hydrolysis renders such an approach commercially impractical. Furthermore, in any such process, cellulase enzyme is significantly diluted because of the low fiber concentration in the mash slurry which results in high loss of enzyme activity over time, and certain components in the mash, such as oil, interfere with cellulase activity.
Accordingly, a method for improving the yield of ethanol from energy crops by converting cellulose and inaccessible starch to fermentable sugar is desirable.